Adjectives with two syllables tend to vary. Some take either form, and the situation determines the usage. For example, one will see commoner and more common, depending on which sounds better in the context. Here are four more examples.
By the way, adjectives of two or more syllables that end in "ow" and "le" also take the "er" and "est" suffixes, e.g., narrower, gentler. Oh, and let's not forget the comparative and superlative of often.
On the flipside, shorter words usually take on the suffixes -er/-est but occasionally they may be given the more/most forms for emphasis or other reasons;e.g., Which of them is the most clear?.
All the best.
- For Teachers